Jovian S-burst observations at 32 MHz

Abstract

JUPITER emits highly sporadic and intense radio emission in the 0.45–40 MHz frequency range¹. Several high-resolution observations below 28 MHz have revealed that a significant fraction of this radiation, perhaps 25%, is comprised of S bursts, which are characterised by rapid negative drift rates R (tens of MHz s⁻¹) and extremely narrow instantaneous band-widths (≤200 kHz^2–7. The probability of observing S bursts, which are associated exclusively with source regions B and C, is determined to a large extent by the geocentric phase of Io. This has led to the widely-held conclusion that the emission is beamed from the planet's Io-associated magnetic flux tube (IFT). S-burst emission models generally assume simple adiabatic motion of trapped electrons accelerated at Io and radiating at or close to the local electron gyrofrequency (refs 8–10). The details of the emission process are unclear and at present offer no restrictions as to the sign of R; however, as the S bursts exhibit negative frequency drift (R<0), we shall speak only in terms of electron bunches radiating as they stream outward from their mirror point near the IFT foot. In this context |R| is predicted to increase rapidly from zero to some maximum value (depending upon electron energy) as the particles execute this motion. In terms of frequency, this region of the northern hemisphere IFT corresponds to f_g ≳ 27 MHz. A determination of R at several points in this frequency range is then a critical test of models incorporating a trapped-electron hypothesis. Accordingly, we present here a preliminary report on observations conducted in this range.